Production of Jet Fuel-Range Hydrocarbons from Hydrodeoxygenation of Lignin over Super Lewis Acid Combined with Metal Catalysts

Article abstract of DOI:10.1002/cssc.201701567

Super Lewis acids containing the triflate anion [e.g., Hf(OTf)₄, Ln(OTf)₃, In(OTf)₃, Al(OTf)₃] and noble metal catalysts (e.g., Ru/C, Ru/Al₂O₃) formed efficient catalytic systems to generate saturated hydrocarbons from lignin in high yields. In such catalytic systems, the metal triflates mediated rapid ether bond cleavage through selective bonding to etheric oxygens while the noble metal catalyzed subsequent hydrodeoxygenation (HDO) reactions. Near theoretical yields of hydrocarbons were produced from lignin model compounds by the combined catalysis of Hf(OTf)₄ and ruthenium-based catalysts. When a technical lignin derived from a pilot-scale biorefinery was used, more than 30 wt % of the hydrocarbons produced with this catalytic system were cyclohexane and alkylcyclohexanes in the jet fuel range. Super Lewis acids are postulated to strongly interact with lignin substrates by protonating hydroxyl groups and ether linkages, forming intermediate species that enhance hydrogenation catalysis by supported noble metal catalysts. Meanwhile, the hydrogenation of aromatic rings by the noble metal catalysts can promote deoxygenation reactions catalyzed by super Lewis acids.

Full text of DOI:10.1002/cssc.201701567

DOI: 10.1002/cssc.201701567
Full Papers
Production of Jet Fuel-Range Hydrocarbons from
Hydrodeoxygenation of Lignin over Super Lewis Acid
Combined with Metal Catalysts
Hongliang Wang,[a, d] Huamin Wang,[b] Eric Kuhn,[c] Melvin P. Tucker,[c] and Bin Yang*[a]
Super Lewis acids containing the triflate anion [e.g., Hf(OTf)4,
Ln(OTf)3, In(OTf)3, Al(OTf)3] and noble metal catalysts (e.g., Ru/
C, Ru/Al2O3) formed efficient catalytic systems to generate sa-
turated hydrocarbons from lignin in high yields. In such cata-
lytic systems, the metal triflates mediated rapid ether bond
cleavage through selective bonding to etheric oxygens while
the noble metal catalyzed subsequent hydrodeoxygenation
(HDO) reactions. Near theoretical yields of hydrocarbons were
produced from lignin model compounds by the combined cat-
alysis of Hf(OTf)4 and ruthenium-based catalysts. When a tech-
nical lignin derived from a pilot-scale biorefinery was used,
more than 30 wt% of the hydrocarbons produced with this
catalytic system were cyclohexane and alkylcyclohexanes in
the jet fuel range. Super Lewis acids are postulated to strongly
interact with lignin substrates by protonating hydroxyl groups
and ether linkages, forming intermediate species that enhance
hydrogenation catalysis by supported noble metal catalysts.
Meanwhile, the hydrogenation of aromatic rings by the noble
metal catalysts can promote oxygenation reactions catalyzed
by super Lewis acids.
Introduction
Current biorefineries undervalue lignin’s potential by burning
it rather than addressing the renewable product requirements
of the world. Utilizing lignin feedstock for the production of
high energy-density jet fuel and value-added chemicals along
with cellulosic ethanol, or other upgradable intermediate
chemicals offers a significant opportunity to enhance the over-
all operational efficiency, carbon conversion efficiency, eco-
nomic viability, and sustainability of biofuels and chemicals
production.[1] The selective conversion of lignin into well-de-
fined products is hampered by several challenges. The first
challenge is derived from lignin’s intrinsic heterogeneous and
robust structure.[2] Lignin is randomly coupled from its three
monolignols (coniferyl, sinapyl and coumaryl alcohols) through
radical processes during construction of the cell wall, which
makes lignin a stable, three-dimensional biopolymer that is
particularly resistant towards biological or chemical decon-
struction and degradation. The second challenge is the high
reactivity of lignin deconstruction intermediates, which are
prone to side reactions to generate stable repolymerized prod-
ucts (e.g., char) through the formation of new CꢀC bonds,
making it difficult to produce high yields of desired products.[3]
The third challenge is principally caused by the complexity of
products from lignin.[4] Unlike cellulose, lignin has no uniform
substructure and linkages. Products from lignin are usually
highly mixed and very complicated, containing hundreds of
compounds with an abundance of functional groups, including
methoxy and hydroxyl groups.[5] The conversion of lignin into
well-defined chemicals can support the emergence of biorefi-
neries that integrate more completely into existing markets
and can be more profitable, supporting larger scale market
penetration.
To fully unlock lignin’s potential, at least the aforementioned
three challenges should be addressed. For the first challenge,
since most of the linkages among lignin’s aromatic units con-
tain CꢀOꢀC bonds,[1c] selective cleavage of these CꢀOꢀC
bonds could effectively depolymerize lignin into aromatic
monomers and dimers.[6] A large proportion of previous re-
search on lignin valorization, including exploring novel cata-
lysts[7] and employing pre-activation approaches (e.g., by selec-
tive pre-oxidation),[8] has been devoted to cleavage of CꢀOꢀC
bonds while avoiding harsh conditions. Several strategies to
address the second challenge have also been attempted, such
as the use of trapping agents, including boric acid,[9] diols,[3b]
phenol, 2-naphthol, and p-cresol,[10] to stabilize the reactive
species and reduce char formation. A feasible approach to ad-
dress the third challenge is to rapidly convert lignin or lignin
deconstruction intermediates into hydrocarbon fuels (e.g., jet
fuel) by using catalytic hydrodeoxygenation (HDO) processing
before additional condensation reactions can occur.[7b,11] Rapid
[a] Dr. H. Wang, Prof. B. Yang
Department of Biological Systems Engineering
Washington State University, Richland, WA 99354 (USA)
E-mail: bin.yang@wsu.edu
[b] Dr. H. Wang
Pacific Northwest National Laboratory
902 Battelle Boulevard, Richland, WA 99354 (USA)
[c] E. Kuhn, Dr. M. P. Tucker
National Bioenergy Center, National Renewable Energy Laboratory
15013 Denver West Parkway, Golden, CO 80401 (USA)
[d] Dr. H. Wang
Current address: Center of Biomass Engineering/College of Agronomy and
Biotechnology
China Agricultural University, Beijing, 100193 (PR China)
The ORCID identification number(s) for the author(s) of this article can
be found under https://doi.org/10.1002/cssc.201701567.
ChemSusChem 2017, 10, 1 – 8
1
ꢀ 2017 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
&
These are not the final page numbers! ÞÞ

Full Papers
HDO process reactions, which involve hydrocracking, hydroge-
nation, and deoxygenation, are necessary to stabilize labile
lignin deconstruction intermediates and convert them into
fuel-range products before condensation reactions can occur.
The use of lignin as a price-competitive source of alternative
jet fuel will help meet the growing worldwide demand for re-
newable jet fuels, while allowing the aviation industry to ach-
ieve carbon-neutral growth. Lignin has a relatively high energy
density and its conversion products with suitable carbon chain
lengths are excellent jet fuel precursors. In addition, conversion
of all major chemical compounds from biomass, including
lignin, offers a significant opportunity for enhancing the overall
profitability of the biorefinery. Despite such potential, the con-
version of lignin into biofuels has proven to be challenging.
Acids combined with metals as bifunctional catalytic systems
have been used for the HDO conversion of lignin into fuels
and have shown very promising performance.[6] However, most
of the acids in these systems are Brønsted acids (e.g., sulfuric
acid, phosphoric acid, H-zeolites). Brønsted acids usually have
low selectivity on lignin chemical bond cleavage, which leads
to low yields of final products.[5a] Lewis acids exhibit different
catalytic performance to Brønsted acids in reactions of CꢀO
bond formation and cleavage.[12] In many cases, they show
better results than Brønsted acids, as they can selectively bond
with and activate specific functional groups (e.g., ether bonds
and hydroxyl groups) during organic transformations.[12] Un-
fortunately, most Lewis acids are water sensitive and can only
be used under strictly anhydrous conditions. Thus, this has
greatly restricted the use of Lewis acids in catalytic reaction-
s.[12a] The exploration of water-tolerant Lewis acids for lignin
conversion is urgently needed.
flates and supported noble metals. The objective of this study
was to identify feasible combinations of catalysts that address
the aforementioned three challenges of lignin utilization in a
one-pot process.
Results and Discussion
Guaiacol was selected as a model compound of lignin to test
the different catalyst systems to simplify product analysis and
provide insight into the reactions. Three different characteristic
CꢀO bonds in guaiacol are common in lignin, that is, Cmethyl
ꢀ
OAr, CarylꢀOMe, and CarylꢀOH. The reactions were carried out at
2508C for 2 h with 4 MPa hydrogen and n-octane as the sol-
vent.
In the absence of catalyst, the conversion of guaiacol was
low (<8 wt%) and most of the products were catechols. In
contrast, 77% of the guaiacol was converted when Ru/Al2O3
catalyst was added. The selectivity to hydrocarbon products
(cyclohexane, alkylcyclohexanes, and dimers of alkylcyclohex-
ane hydrocarbons) was about 40% whereas the selectivity to
oxy-compounds (mainly cyclohexanol and cyclohexane-1,2-
diol) was close to 60%, suggesting that the catalytic deoxyge-
nation activity of Ru/Al2O3 is not high. No aromatic products
were detected, indicating Ru/Al2O3 had a high aromatic ring
hydrogenation catalytic activity under the tested reaction con-
ditions that led to full aromatic ring saturation. Subsequently,
several metal triflates were combined separately with Ru/Al2O3
with the aim to enhance the HDO performance. The conver-
sion of guaiacol, as well as the selectivity of hydrocarbon prod-
ucts significantly increased when Hf(OTf)4, Al(OTf)3, or In(OTf)3
were added (Table 1, entries 3–6), especially when Hf(OTf)4 was
used, because almost all of the guaiacol was selectively con-
verted into hydrocarbon products.
Metal triflates are widely used as novel super Lewis acids in
organic synthesis, and they are relatively inexpensive and ther-
mally stable, and can be economically recycled.[12b,13] Moreover,
they are water insensitive, which can keep the catalyst struc-
ture stable in water while maintaining the active sites. Several
research groups have reported that metal triflates are effective
for biomass conversion, especially for lignin depolymeriza-
tion.[14] Metal triflates with strong Lewis acidities have been
demonstrated effective in catalyzing cleavage of lignin b-O-4
ether bonds, a major ether bond in lignin cross-linkages.[3b,15]
The strong electron-withdrawing ꢀOTf group (CF3SO3ꢀ) can
make the metals in metal triflates very cationic. The cationic
metals can selectively bond with electron-rich atoms (e.g.,
oxygen atoms in b-O-4 and a-O-4 ether bonds) and promote
the cleavage of related chemical bonds.[14a,c] Moreover, the
phenolic hydroxyl groups in lignin can be exchanged with
ꢀOTf groups, facilitating the removal of oxygen on aromatic
rings.
Hf(OTf)4, as a strong Lewis acid, was tested alone in the con-
version of guaiacol. Interestingly, guaiacol conversion was not
high (58%) and oxy-compounds (phenols and alkylphenols)
were detected as the major products. It should be noted that
the oxy-compounds (aromatics) obtained by using Hf(OTf)4
were different from those (cyclohexanols) formed by using Ru/
Al2O3 catalysis. These results suggest that Ru/Al2O3 is a catalyst
not only for hydrogenation reactions, but also partly for
oxygen-removal reactions. It has also been demonstrated that
the hydrogenation of aromatic rings is favorable for the re-
moval of ring-associated oxy groups.[16] Usually, sp2 hybrid
CaromaticꢀO bonds have 80–100 kJmolꢀ1 higher bond dissocia-
tion energy than sp3 hybrid CꢀO bonds. The hydrogenation of
the aromatic rings can destroy the aromaticity of the reactant
and change the sp2 hybrid CaromaticꢀO bonds into sp3 hybrid Cꢀ
O bonds and thus facilitate the removal of oxygen on aromatic
rings. Some dimer products were detected by using Hf(OTf)4
only, which were also aromatics. The total selectivity of the GC-
MS detectable products catalyzed by Hf(OTf)4 was less than
100%, indicating that some high molecular weight products
existed, most probably produced from condensation and poly-
merization reactions. When no Ru-based catalysts were pres-
ent, the conversion of lignin deconstruction intermediates into
char instead of hydrocarbon products increased, since these in-
We report herein that metal triflates can be used to replace
conventional Brønsted/Lewis acids for the HDO conversion of
lignin into hydrocarbons with supported metal cocatalysts.
Metal triflates, with different Lewis acidities, were tested in the
catalytic systems to pinpoint the catalytic nature of these ma-
terials. Several lignin model compounds and technical lignins
were employed as reactants to get additional insights into
lignin HDO conversion by the combined catalysis of metal tri-
&
ChemSusChem 2017, 10, 1 – 8
www.chemsuschem.org
2
ꢀ 2017 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
ÝÝ These are not the final page numbers!

Full Papers
Table 1. HDO conversion of guaiacol with different metal triflate/supported noble metal catalyst systems.[a]
Entry
Catalyst
Conversion
[wt%]
Product selectivity [C%]
hydrocarbon
Total hydrocarbon yield
[C%]
hydrocarbon
monomers
oxy-compounds
dimers
1
2
3
4
5
6
7
8
none
<8
77
86
N.D.
38.2
49.9
78.1
82.6
71.9
45.6
46.6
2.0
75.0
65.9
84.5
55.2
60.3
N.D.
2.4
3.6
10.2
17.4
18.7
3.1
4.8
6.3
13.7
14.6
15.5
2.3
96.0
59.4
46.5
11.7
N.D.
9.4
51.3
48.6
68.0
11.3
19.5
N.D.
42.5
31.8
N.D.
Ru/Al2O3 only
Ni(OTf)2+Ru
Al(OTf)3+Ru
Hf(OTf)4+Ru
In(OTf)3+Ru
ZnCl2+Ru
31.3
46.0
80.4
>99.9
91.0
40.4
44.7
1.2
88.7
72.1
>99.9
48.3
59.3
91
>99
>99
83
87
58
>99
89
>99
84
AlCl3+Ru
9
Hf(OTf)4 only
Hf(OTf)4+Pd
Hf(OTf)4+Pt
Hf(OTf)4+Ru
H3PO4+Ru
HY+Ru
10
11
12[b]]
13[c]
14[d]
87
7.9
[a] Reaction conditions: guaiacol (2.5 mmol), metal triflate (1 mol%), M (0.8 mol% as M/Al2O3; M=Ru, Pd, or Pt), n-octane (1 mL), T=2508C, t=2 h, PH
4 MPa. [b] 0.8 mol% Ru as 5 wt% Ru/C. [c] 1 mol% H3PO4. [d] 20 mg zeolite HY. N.D.=not detected.
=
2
termediates are aromatic oxy-compounds with high polymeri-
zation reactivity under Lewis acid catalysis, increasing CꢀC
bond formation through alkylation and aldol condensation re-
actions.
than AlCl3. Metals in strong Lewis acids have a strong interac-
tion with oxygen atoms in ether bonds, and they can effective-
ly activate and cleave these bonds. The effective cleavage of
ether bonds by strong Lewis acids can form intermediate spe-
cies that can be converted on Ru/Al2O3 more efficiently
through hydrogenation and deoxygenation reactions. Brønsted
acids, including H3PO4 and HY zeolite, were also used to re-
place metal triflates. Both guaiacol conversion and hydrocar-
bon product yields were low. The different performances of
Brønsted acids to Lewis acids are probably because Lewis
acids can selectively interact with oxygens in the reactant, ach-
ieving highly efficient cleavage of the ether bond and oxygen
removal with the accompaniment of Ru/Al2O3 hydrodeoxyge-
nation catalysis.
Unlike Hf(OTf)4, Al(OTf)3, or In(OTf)3, the addition of Ni(OTf)2
showed minimal improvement in guaiacol HDO conversion,
with half of the products being oxy-compounds. The differen-
ces in the catalytic behaviors of the tested metal triflates may
lie in the differences of the effective charge density of their
metal ions.[13b,14d] In general, the Lewis acidity of metal triflates
increased with the effective charge density of their central
metal cations. The effective charge density of the metal ions in
Hf(OTf)4, Al(OTf)3, and In(OTf)3 is higher than that in Ni(OTf)2,
which means that the Lewis acidity of Hf(OTf)4, Al(OTf)3, and
In(OTf)3 is stronger than that of Ni(OTf)2. Catalysts with higher
Lewis acidities are more effective in activating chemical bonds,
for example, CꢀO bonds, and achieve highly efficient cleavage
of these bonds through proton transfers. To further verify this
hypothesis, metal chlorides, including ZnCl2 and AlCl3, were
used in combination with Ru/Al2O3 to test the hypothesis.
Results were unsatisfactory. The conversion of guaiacol was in-
complete and almost half of the products were oxy-com-
pounds. Although AlCl3 and Al(OTf)3 have the same cation and
a similar pH in solution, their catalytic results are quite differ-
ent, indicating that neither cation nor Arrhenius acidity is the
decisive factor affecting the reactivity. The effective charge
density of metal cations plays an important role in the reac-
tions. Anions in Lewis acids could influence the effective
charge densities of the central metal ions, and thus influence
the Lewis acidity, which relates to the strengths of the sub-
strate–catalyst interactions. Al3+ in Al(OTf)3 is more cationic
than it is in AlCl3, so the Lewis acidity of Al(OTf)3 is stronger
As indicated above, both strong Lewis acids and Ru/Al2O3
are essential for achieving high yields of hydrocarbons in
guaiacol HDO conversions. Ru-based catalysts have been
widely used for lignin conversion. Ru, as compared with other
noble metals, is less expensive and has superior HDO catalytic
activity.[17] In addition, other noble metals commonly used in
hydrogenation reactions were explored to replace Ru/Al2O3.
Pd/Al2O3 and Pt/Al2O3 combined with Hf(OTf)4 were tested in
the reactions (Table 1, entries 10–12). Both supported noble
metal catalysts improved their catalytic behavior in the pres-
ence of Hf(OTf)4. The conversion of guaiacol and the selectivity
to deoxygenated products were close to those when catalyzed
by Ru/Al2O3. Moreover, other supports besides Al2O3 were also
tested in this reaction. Activated carbon (C) was chosen to re-
place Al2O3, which has a degree of Lewis acidity. The loading
of Ru in Ru/C was the same as that in Ru/Al2O3 and similar
HDO results were obtained.
ChemSusChem 2017, 10, 1 – 8
www.chemsuschem.org
3
ꢀ 2017 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
&
These are not the final page numbers! ÞÞ

Full Papers
Effects of solvents on guaiacol HDO conversion were investi-
gated next (Figure 1). Guaiacol conversion was nearly 90% and
the yield of hydrocarbon products was about 60% when no
solvent was added. About 10 mol% of oxy-compounds (mainly
Ether bonds in methoxy groups, as well as in a-O-4 (entry 2)
and b-O-4 linkages (entry 4), were completely cleaved. Almost
all oxygen atoms in these model compounds were removed.
High yields of hydrocarbon products were obtained from all of
the tested model compounds. Moreover, most of the ring
structures, side carbon chains, and carbon linkages between
aromatic rings (Table 2, entry 5) in these reactants remained
after the reactions, indicating that the catalytic systems had
little effect on the carbon skeleton.
The HDO conversion of corn stover lignin obtained from
dilute alkali deacetylation and mechanical refining (DMR) treat-
ment, an alternative biomass deconstruction process devel-
oped at the National Renewable Energy Laboratory (NREL),
was investigated. This technology has been shown to produce
highly digestible pretreated solid residues and result in high-
concentration sugar syrups (ca. 230 gLꢀ1 fermentable mono-
meric sugars) and high sugar yields (up to 90% cellulose to
glucose) with commercial enzyme preparations at high solid
loadings (up to 30 wt% insoluble solids).[6a] Potential value-
added streams produced in the DMR process are the “native-
like” and tractable lignin residues isolated from the deacetyla-
tion process and enzymatic hydrolysis. These lignin streams
can be converted into fuels and chemicals instead of being
burned to produce electricity, thus increasing a biorefinery’s
revenues. A large portion (ca. 80%) of the lignin is carried all
the way through enzymatic hydrolysis as insoluble solids in the
DMR process. After purification, the DMR lignin was character-
ized by using 2D NMR spectroscopy. The inter-unit linkages
among the aromatic units of the DMR lignin were found to be
about 43, 44, 9, and 4% for b-O-4, b-5, b–b, and b-1, respecti-
vely.[6a]
Figure 1. Effects of solvents on the HDO conversion of guaiacol. Reaction
conditions: guaiacol (2.5 mmol), Hf(OTf)4 (1 mol%), Ru (0.8 mol%; as 5 wt%
Ru/Al2O3), solvent (1 mL), T=2508C, t=2 h, PH =4 MPa. Control: guaiacol
2
(5 mmol), Hf(OTf)4 (1 mol%), Ru (0.8 mol%; as 5 wt% Ru/Al2O3), no solvent.
n-octane +water means 90 wt% n-octane mixed with 10 wt% water.
cyclohexanol and 2-cyclohexylcyclohexanol) were obtained. No
other products were detected by GC-MS. The conversion of
guaiacol and the yield of hydrocarbon products were quite
low when the reactions were carried out in water. This result is
probably due to the low solubility of guaiacol in water and the
instability of Ru/Al2O3 in hot water. Although methanol and di-
chloromethane led to relatively high conversion of guaiacol,
the yields of hydrocarbons in these solvents were low. Prod-
ucts, mainly various oxy-compounds, obtained in water, meth-
anol, and dichloromethane were quite complicated. n-Octane
was found to be an excellent solvent for the reactions. Guaia-
col was completely converted into hydrocarbons in n-octane.
The use of a suitable solvent such as n-octane was found to be
better than the control reactions for the following reasons:
i) Catalysts, temperature, and pressure are more stable within
solvents; ii) the dissolution and transportation of H2 are superi-
or; iii) these solvents hold the reactant, intermediates, and
products in solution.[13b] Since it is difficult and costly to
remove all water from biomass feedstocks, 10 wt% of water
was mixed with n-octane as the mixed solvent, whereby ap-
proximately 100% guaiacol conversion was observed and the
yield of hydrocarbons was acceptable. The good performance
of these catalyst systems in n-octane may result from greater
H2 solubility, better heat and mass transfer, and less adverse
impact to the catalysts (e.g., non-active for competitive ad-
sorption). The use of n-octane or other inexpensive hydrocar-
bon solvents is especially beneficial for the current catalytic
systems since these hydrocarbons can be directly used as fuel
without expensive separation.
The HDO conversion of DMR lignin was initially tested with
Ru/Al2O3 as catalyst at 2508C under 4 MPa hydrogen pressure
for 4 h (Figure 2a). The yield of hydrocarbon products was low
(<5 wt%). An additional large portion of products was found
to be oxy-compounds, mainly including the derivatives of cy-
clohexanone and cyclohexanol. When Hf(OTf)4 was used alone,
the yield of hydrocarbon products was negligible (Figure 2b).
The majority of the products from DMR lignin by the catalysis
of Hf(OTf)4 was found to be phenols and alkylphenols. More
Under the reaction conditions of guaiacol conversion report-
ed herein, several other lignin model compounds with a broad
range of functional groups and linkages were tested. Most of
the CꢀO bonds, including the ether bonds and CꢀOH bonds,
were cleaved efficiently, although a small amount of CꢀOꢀC
bonds in diphenyl ether remained stable (Table 2, entry 3).
Figure 2. Hydrocarbon yields from DMR lignin in HDO conversion under dif-
ferent conditions. Reaction conditions: lignin (50 mg), n-octane (1 mL),
T=2508C, t=4 h, PH =4 MPa. Catalysts: a) 10 mg Ru/Al2O3. b) 15 mg
2
Hf(OTf)4. c) 10 mg Ru/Al2O3 +15 mg Hf(OTf)4. d) 10 mg Ru/Al2O3 +15 mg
Hf(OTf)4 in 1 mL n-octane containing 10 wt% water as solvent. e) No catalyst
was added.
&
ChemSusChem 2017, 10, 1 – 8
www.chemsuschem.org
4
ꢀ 2017 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
ÝÝ These are not the final page numbers!

Full Papers
Table 2. HDO conversion of other lignin model compounds by the combined catalysis of Hf(OTf)4 and Ru/Al2O3.[a]
Entry Reactant
Conversion
[wt%]
Selectivity [C%]
Total hydrocarbon yield
[C%]
1
>98
>98
>99
>99
>99
70.2
45.0
11.5
20.1
5.5
5.6
4.8
2
11.6
3
4
94.3
87.2
87.5
37.8
4.3
4.1
3.6
4.5
15.4
12.3
>98
5
>99
>99
97.1
2.2
[a] Reaction conditions: 2.5 mmol lignin model compounds, 1 mol% Hf(OTf)4, 0.8 mol% Ru as 5 wt% Ru/Al2O3, 1 mL n-octane. T=2508C, t=2 h, PH
=
2
4 MPa.
than 30 wt% of hydrocarbon products was obtained from
DMR lignin HDO conversion by using the combined catalysis
of Ru/Al2O3 and Hf(OTf)4. This yield was about six times that
obtained by using Ru/Al2O3 alone, demonstrating that Hf(OTf)4
provided significant synergistic effects with Ru/Al2O3 in the
conversion of technical lignin into hydrocarbons. Moreover,
the majority of the hydrocarbon products produced with the
combined catalysts was found to maintain ring structures and
mainly contained 9–18 carbon atoms, suitable for fuel usage
(Figure 3). Additionally, when mixed solvents containing
90 wt% n-octane and 10 wt% water was used to replace pure
n-octane, the yield of hydrocarbon products remained similar.
Negligible amount of hydrocarbon products was generated
when no catalyst was added (Figure 2e). The yield of other
lignin depolymerization products, such as monomer and dimer
aromatics, was also quite low (<10 wt%). The stability and re-
usability of metal triflates were proven very good.[13b,14a]
A slight efficiency loss in hydrocarbon production from lignin
was found when the catalysts were reused, and this was prob-
ably due to the formation of some clusters in the Ru-based
catalyst.
Figure 3. GC-MS spectrum of DMR lignin HDO products from the combined
catalysis of Ru/Al2O3 and Hf(OTf)4.
ChemSusChem 2017, 10, 1 – 8
www.chemsuschem.org
5
ꢀ 2017 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
&
These are not the final page numbers! ÞÞ

Full Papers
fuged. The methanol phase was combined in the volumetric flask.
A small quantity of 3-methylheptane (0.2 mmol) was added into
the diluted liquid solution and used as the internal standard in GC
analysis.
Conclusions
High yields of hydrocarbons with the carbon numbers in the
jet fuel range were obtained from technical lignin or lignin
model compounds by the combined catalysis of strong Lewis
acids [e.g., Hf(OTf)4, Al(OTf)3, or In(OTf)3] and supported ruthe-
nium catalysts (e.g., Ru/Al2O3 or Ru/C) in a simple one-pot pro-
cess. Almost all of the CꢀO bonds, including ether bonds and
CꢀOH bonds, were found to be cleaved in the tested lignin
model compounds. Meanwhile, oxygen in these compounds
was efficiently removed while the carbon skeletal structures
were retained. Sufficient evidence is presented to show that
strong Lewis acids had synergistic effects in combination with
Ru/Al2O3 on HDO conversion of lignin and its model com-
pounds. The HDO reactions in these catalytic systems were en-
hanced by using n-octane as solvent. These results highlight
the potential for an efficient HDO catalytic process to produce
jet fuel from biorefinery lignin waste streams.
Analysis of HDO products
The methanol-diluted liquid samples were analyzed by GC and GC-
MS in an Agilent Technologies 7890A GC system with a DB-5 capil-
lary column (30 m lengthꢁ250 mm I.D.ꢁ0.25 mm film thickness;
J&W Scientific) in the splitless mode. Typically, a 1 mL sample was
injected with 0.6 mLminꢀ1 of He used as the carrier gas into the
GC system. The injection port temperature was set at 3008C. The
GC oven was programmed to 328C for 10 min. Then the tempera-
ture was raised at a rate of 108Cminꢀ1 until it reached 3008C and
was held at this temperature for 2 min. Eluting compounds were
detected with a MS (Agilent Technologies 5975C) inert XL EI/CI
MSD with a triple axis detector, and compared by using NIST libra-
ries. The calculations of conversion and selectivity of lignin model
compounds were based on carbon, and the calculations of conver-
sions and selectivity of technical lignin were based on weight.
For lignin model compounds [Eqs. (1)–(3); HC=hydrocarbon]:
Experimental Section
Chemicals and materials
Reactant converted
Reactant added
ð1Þ
ð2Þ
ð3Þ
Conversion % ¼
ꢁ 100%
All the chemicals used in this research are commercially available
and used as received without further purifications. Metal triflates,
including Hf(OTf)4, Ln(OTf)3, In(OTf)3, Al(OTf)3, and Ni(OTf)2 (OTf=
trifluoromethanesulfonate group), were purchased from Sigma–Al-
drich. Reduced catalysts, including Ru/Al2O3, Pd/Al2O3, Pt/Al2O3, and
Ru/C, were purchased from Alfa Aesar. Lignin model compounds,
including guaiacol, 4-ethyl-2-methoxyphenol, benzyloxy benzene,
diphenyl ether, and 1,1’-biphenyl, were purchased from Fisher Sci-
entific. Lignin b-O-4 model compounds were purchased from
GreenLignol, LLC. Zeolite HY (CBV 400) was purchased from Zeolyst
International. Corn stover lignin was obtained by the National Re-
newable Energy Laboratory (NREL) through dilute alkali deacetyla-
tion and mechanical refining (DMR) treatment. All other chemicals
were purchased from Fisher Scientific.
C atoms in HC A
C atoms in reactant
Yield of HC A % ¼
ꢁ 100%
C atoms in HC A
Total C atoms in products
Selectivity to HC A % ¼
ꢁ 100%
For the conversion of technical lignin [Eqs. (4)–(6)]:
Weight of reactant converted
Weight of reactant added
ð4Þ
ð5Þ
Conversion % ¼
ꢁ 100%
Weight of HC A produced
Weight of reactant added
Yield of HC A % ¼
ꢁ 100%
20
X
Total HC yield % ¼
YieldðxÞ ꢁ 100%
ð6Þ
x¼1
Hydrodeoxygenation conversion of lignin model com-
pounds and lignin
Metal triflate recycling
In a typical reaction, lignin (50 mg) or lignin model compounds
(2.5 mmol), catalysts, and solvents (1 mL), as needed, were added
to a 3 ml dry glass sleeve. The sleeve was placed into a high-
throughput batch reactor (PNNL-SA-117072) at the Bioproducts,
Science & Engineering Laboratory. The reactor was sealed and
purged with H2 three times to exclude air, and then pressurized
with 4 MPa H2 at room temperature. The reactor was heated to
2508C and heating was maintained for 2 or 4 hours, depending on
the substrate. The metal plate of the high-throughput reactor was
shaken at a rate of 80 countsminꢀ1 during the reaction to improve
After reaction, solvents and hydrocarbon products were removed
from the reaction mixture by rotary evaporation. The residue was
separated by extraction with water and ethyl acetate. Most of the
initial metal triflates was found in the water phase. After filtration,
water was removed from the filtrate by rotary evaporation under
vacuum. The residue (mainly metal triflate) was redissolved in n-
octane and used in a new reaction under standard conditions.
the mass transfer. After each run, the reactor plate was cooled to Acknowledgements
room temperature to terminate the reaction by removing the heat.
(WARNING! Use caution when handling and venting the reactor
This work was supported by the National Renewable Energy Lab-
and glass sleeves; the reaction mixtures containing Ru/C and or-
ganic solvents are extremely catalytically reactive, flammable, and
pyrophoric when exposed to air). The glass sleeve was removed
from the high-throughput reactor and the liquids were separated
from the solids by centrifugation (8000 rminꢀ1 for 10 min). The
liquid was diluted with methanol in a 10 mL volumetric flask. The
solids were washed with an additional 10 mL methanol and centri-
oratory Subcontract # AEV-6–52054-01 under Prime U.S. Depart-
ment of Energy (DOE) Award # DE-AC36-08G028308, and the Sun
Grant-U.S. Department of Transportation (DOT) Award # T0013G-
A-Task 8 with the Bioproducts, Science & Engineering Laboratory
and Department of Biological Systems Engineering at Washing-
ton State University. This work was performed in part at the Wil-
&
ChemSusChem 2017, 10, 1 – 8
www.chemsuschem.org
6
ꢀ 2017 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
ÝÝ These are not the final page numbers!

Full Papers
[7] a) T. H. Parsell, B. C. Owen, I. Klein, T. M. Jarrell, C. L. Marcum, L. J. Hau-
pert, L. M. Amundson, H. I. Kenttaemaa, F. Ribeiro, J. T. Miller, M. M. Abu-
Omar, Chem. Sci. 2013, 4, 806–813; b) R. Ma, W. Y. Hao, X. L. Ma, Y. Tian,
Y. D. Li, Angew. Chem. Int. Ed. 2014, 53, 7310–7315; Angew. Chem. 2014,
126, 7438–7443; c) Q. Song, F. Wang, J. Y. Cai, Y. H. Wang, J. J. Zhang,
W. Q. Yu, J. Xu, Energ. Environ. Sci. 2013, 6, 994–1007.
[8] a) C. S. Lancefield, O. S. Ojo, F. Tran, N. J. Westwood, Angew. Chem.
2015, 127, 260–264; b) A. Rahimi, A. Ulbrich, J. J. Coon, S. S. Stahl,
Nature 2014, 515, 249–252.
liam R. Wiley Environmental Molecular Sciences Laboratory
(EMSL), a national scientific user facility sponsored by the U.S.
DOE’s Office of Biological and Environmental Research and locat-
ed at the Pacific Northwest National Laboratory, operated for the
DOE by Battelle. The authors would like to thank Ms. Heather Job
and Ms. Marie S. Swita, who helped us to conduct the high-
throughput experiments and to collect part of the GC-MS data
for this project.
[9] V. M. Roberts, V. Stein, T. Reiner, A. Lemonidou, X. Li, J. A. Lercher, Chem.
Eur. J. 2011, 17, 5939–5948.
[10] J. B. Li, G. Henriksson, G. Gellerstedt, Bioresour. Technol. 2007, 98, 3061–
3068.
Conflict of interest
[11] a) X. Y. Wang, R. Rinaldi, Angew. Chem. Int. Ed. 2013, 52, 11499–11503;
Angew. Chem. 2013, 125, 11713–11717; b) Q. Xia, Z. Chen, Y. Shao, X.
Gong, H. Wang, X. Liu, S. F. Parker, X. Han, S. Yang, Y. Wang, Nat.
Commun. 2016, 7, 11162; c) X. Cui, A.-E. Surkus, K. Junge, C. Topf, J.
Radnik, C. Kreyenschulte, M. Beller, Nat. Commun. 2016, 7, 11326.
[12] a) S. Kobayashi, K. Manabe, Acc. Chem. Res. 2002, 35, 209–217; b) T. L.
Lohr, Z. Li, T. J. Marks, Acc. Chem. Res. 2016, 49, 824–834.
The authors declare no conflict of interest.
Keywords: biomass conversion
· fuels · hydrocarbons ·
hydrodeoxygenation · metal triflates
[13] a) Z. Li, R. S. Assary, A. C. Atesin, L. A. Curtiss, T. J. Marks, J. Am. Chem.
Soc. 2014, 136, 104–107; b) H. J. Song, J. Deng, M. S. Cui, X. L. Li, X. X.
Liu, R. Zhu, W. P. Wu, Y. Fu, ChemSusChem 2015, 8, 4250–4255.
[14] a) R. Jastrzebski, S. Constant, C. S. Lancefield, N. J. Westwood, B. M.
Weckhuysen, P. C. Bruijnincx, ChemSusChem 2016, 9, 2074–2079; b) P. J.
Deuss, C. W. Lahive, C. S. Lancefield, N. J. Westwood, P. C. Kamer, K.
Barta, J. G. de Vries, ChemSusChem 2016, 9, 2974–2981; c) L. Yang, Y. Li,
P. E. Savage, Ind. Eng. Chem. Res. 2014, 53, 2633–2639; d) R. S. Assary,
A. C. Atesin, Z. Li, L. A. Curtiss, T. J. Marks, ACS Catal. 2013, 3, 1908–
1914; e) J. Degenstein, S. R. Kamireddy, M. P. Tucker, Y. Ji, Int. J. Agric.
Biol. Eng. 2013, 6, 54–62.
[15] X. Huang, O. M. Morales Gonzalez, J. Zhu, T. I. Koranyi, M. D. Boot,
E. J. M. Hensen, Green Chem. 2017, 19, 175–287.
[16] a) X. Zhu, L. L. Lobban, R. G. Mallinson, D. E. Resasco, J. Catal. 2011, 281,
21–29; b) H. Wang, M. Feng, B. Yang, Green Chem. 2017, 19, 1668–
1673.
[1] a) A. J. Ragauskas, G. T. Beckham, M. J. Biddy, R. Chandra, F. Chen, M. F.
Davis, B. H. Davison, R. A. Dixon, P. Gilna, M. Keller, P. Langan, A. K.
Naskar, J. N. Saddler, T. J. Tschaplinski, G. A. Tuskan, C. E. Wyman, Science
2014, 344, 1246843; b) D. D. Laskar, B. Yang, H. Wang, J. Lee, Biofuels Bi-
oprod. Biorefin. 2013, 7, 602–626; c) J. Zakzeski, P. C. A. Bruijnincx, A. L.
Jongerius, B. M. Weckhuysen, Chem. Rev. 2010, 110, 3552–3599.
[2] a) S. Stiefel, C. Marks, T. Schmidt, S. Hanisch, G. Spalding, M. Wessling,
Green Chem. 2016, 18, 531–540; b) T. Saito, J. H. Perkins, F. Vautard,
H. M. Meyer, J. M. Messman, B. Tolnai, A. K. Naskar, ChemSusChem 2014,
7, 221–228.
[3] a) L. Shuai, M. T. Amiri, Y. M. Questell-Santiago, F. Hꢂroguel, Y. Li, H. Kim,
R. Meilan, C. Chapple, J. Ralph, J. S. Luterbacher, Science 2016, 354,
329–333; b) P. J. Deuss, M. Scott, F. Tran, N. J. Westwood, J. G. de Vries,
K. Barta, J. Am. Chem. Soc. 2015, 137, 7456–7467.
[4] a) C. W. Lahive, P. J. Deuss, C. S. Lancefield, Z. Sun, D. B. Cordes, C. M.
Young, F. Tran, A. M. Slawin, J. G. de Vries, P. C. Kamer, J. Am. Chem. Soc.
2016, 138, 8900–8911; b) S. Van den Bosch, W. Schutyser, R. Vanholme,
T. Driessen, S. F. Koelewijn, T. Renders, B. De Meester, W. Huijgen, W.
Dehaen, C. Courtin, Energ. Environ. Sci. 2015, 8, 1748–1763.
[17] a) C. Michel, P. Gallezot, ACS Catal. 2015, 5, 4130–4132; b) J. Zhang, J.
Teo, X. Chen, H. Asakura, T. Tanaka, K. Teramura, N. Yan, ACS Catal. 2014,
4, 1574–1583.
[5] a) C. Li, X. Zhao, A. Wang, G. W. Huber, T. Zhang, Chem. Rev. 2015, 115,
11559–11624; b) H. Wang, T. Deng, H. Ruan, X. Hou, J. R. Cort, B. Yang,
Green Chem. 2016, 18, 2802–2810.
Manuscript received: August 19, 2017
Revised manuscript received: October 11, 2017
Version of record online: && &&, 0000
[6] a) H. Wang, H. Ruan, H. Pei, H. Wang, X. Chen, M. P. Tucker, J. R. Cort, B.
Yang, Green Chem. 2015, 17, 5131–5135; b) N. Yan, C. Zhao, P. J. Dyson,
C. Wang, L. T. Liu, Y. Kou, ChemSusChem 2008, 1, 626–629.
ChemSusChem 2017, 10, 1 – 8
www.chemsuschem.org
7
ꢀ 2017 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
&
These are not the final page numbers! ÞÞ

FULL PAPERS
H. Wang, H. Wang, E. Kuhn, M. P. Tucker,
B. Yang*
The biggest Lewiser: Super Lewis acids
and noble metals form an efficient cata-
lytic system that can overcome the
energy barrier for conversion of lignin
into high yield jet-fuel range hydrocar-
bons. Metal triflates mediate rapid ether
bond cleavage through selective bond-
ing to the etheric oxygen, whereas the
noble metal catalyzes the subsequent
hydrogenation reaction, eliminating
functional groups.
&& – &&
Production of Jet Fuel-Range
Hydrocarbons from
Hydrodeoxygenation of Lignin over
Super Lewis Acid Combined with
Metal Catalysts
&
ChemSusChem 2017, 10, 1 – 8
www.chemsuschem.org
8
ꢀ 2017 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
ÝÝ These are not the final page numbers!